Most glider radio use these days is glider-to-glider, on 130.1, 130.125 or 130.4. Calls to the ground when they happen are mostly for final glide [= impending arrival]. As over 1000 gliders may be active at a time, more in the South East/midlands than anywhere else, all having to share these three frequencies, they are busy. Some calls have sufficient urgency - e.g. when entering cloud, using 130.4 - that I and many other pilots will call out even if it creates simultaneous transmission with others - the theory being that via the inverse square law, any glider close enough to be at risk of being in the same cloud will get the transmission in preference to calls from further away.
That is one set of reasons why calls from power planes enquiring about the presence of gliders would not be welcomed.
Another reason is that even if you call, and get an answer which sounds useful to you, it is dangerously misleading. There could be gliders where you don't hear about them being around, as well as where you do hear about. Looking too much in one direction means less in other directions.
Because of the large amount of chat, many glider pilots switch off except when they need to communicate.
Some people - I am one - listen more on ATC frequencies for part of the time. I do so when flying near my home site, Ridgewell, just outside the Stansted airspace, because lots of power planes skirt the zone and hence come right over us (some idiots fly over the airfield at less than winch launch height, in blatant disregard of the Rules of the Air - one day there will be a fatality here, like the one at another site a few years ago). Occasionally, though less with the latest Stansted airspace, we have had airliners in class G. So I often listen out on 120.62 for potential conflict with heavy metal. I rarely call Essex Radar, because past experience is that they cheerfully talk to power planes but just tell gliders to stay clear and don't want to hear any more, even if (as in my last attempt) it is to tell them they are working a power plane in class G straight at me, the power pilot having informed them that he could see no gliders.
So, trying to rely on radio for reducing chances of conflict with gliders is perhaps counterproductive. The mark one eyeball is imperfect, but it may be the best we all have to rely upon, until the invention and widespread use of some sort of effective proximity warning system for gliders/GA.
Chris N.